PHP class not found but it's included

Check to make sure your environment isn't being picky about your opening tags. My configuration requires:

<?php

If i try to use:

<?

Then I get the same error as you.

if ( ! class_exists('User')) 
    die('There is no hope!');

I had this problem and the solution was namespaces. The included file was included in its own namespace. Obvious thing, easy to overlook.